This invention relates generally to toothed belts for power transmission and to a method for producing the same. The invention has particular utility in a toothed belt for driving the camshaft of an automobile engine.
A conventional toothed belt is composed of a tension bearing member, e.g. a set of core wires, sandwiched between a rubber tooth portion, i.e. a "tooth rubber", and a rubber back portion, i.e. a "back rubber". The outer surface of the tooth rubber is covered by a tooth cloth, which usually comprises a polyamide (nylon).
In the manufacture of the belt, the tooth cloth is typically treated with a rubber paste the composition of which is essentially the same as that of the tooth rubber and the back rubber. Alternatively, the tooth rubber is treated with a resorcinol formaldehyde latex (RFL) solution including VP-SBR latex, i.e. vinyl pyridine--styrene butadiene rubber latex. As used herein, the term "treated" includes "being impregnated", "being submerged and thereafter dried", "being coated and thereafter dried" and equivalent operations. The tooth cloth treated with RFL solution may be further treated with a rubber paste. In the past, the tooth rubber and back rubber have typically been composed of CR (chloroprene). However, more recently, primarily as a result of the tendency toward increased horsepower in automobile engines, CR (chloroprene) has been displaced by HSN (hydrogenated nitrile) as the material of the tooth rubber and back rubber.
Because automobiles are essential and are becoming more and more sophisticated, maintenance-free engine parts are highly desirable. A toothed belt is an engine component which, in the past, has been especially subject to failure. A toothed belt having a long service life is therefore particularly desirable.
To improve the service life of a toothed belt, it is essential to identify the cause of belt failure. A toothed belt usually fails as it is severed or as its teeth break. The primary object of this invention is to improve the breaking strength of the teeth in a toothed belt.
Past efforts to improve the breaking strength of the teeth in a toothed belt have generally employed means to improve the anti-wear and heat-resisting properties of the tooth cloth. Efforts to improve the anti-wear properties have included increasing the amount of fiber per unit area of the tooth cloth, and improving the anti-wear properties of the fibers used in the tooth cloth. To improve the heat resisting properties of a tooth cloth, the cloth has been treated with HSN (hydrogenated nitrile) rubber paste.